Calculating drapery yardage need not become complicated. I recommend you work in either inches or cms, and then stick with that for all your calculations.
Here's how to do it, step by step.
Allowances for headings (looking down on track)
You need to add an allowance to the track or pole width to get the heading width.
If you make each drape heading to exactly one half of the track length, then the drapes will pull back from each other leaving a gap (A).
By adding 3" (8cms) to each drape, they will have enough take up to prevent gapping. Also, if the track incorporates 'overlaps', the headings will be wide enough. (B) If your track goes back to the wall (C) then add another 3" (8cms) per drape.
If there are no returns (A & B), add 6" (15cms) to the track or pole length.
If there are returns (C), add 12" (30cms) to the track or pole length.
Reduce the fabric width by 3" (8cms). This is to allow for selvages and joins.
The fullness ratio is the amount of 'gather' in your drapes. If you use styling tape (heading tape) the manufacturer will indicate what fullness you should use.
If you're hand heading (which gives the best results in my opinion), then you have more leeway. Fullness will vary between 2.0 and 2.5, occasionally going to 3.0. I suggest you don't go below a fullness of 2.3. You'll often find the fullness doesn't work out exactly to the value you want. Always go for extra fullness if in doubt.
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Multiply the Total Heading by the Fullness ratio.
Divide the result by the Useable width of the fabric, and round up to the nearest width.
The hook is set down from the top of the heading (yellow arrow). This is the difference between the Finished Length (the top red line to bottom of drape) and the Hook Finish (the bottom red line to bottom of drape).
Hook finish is the length of the drapes, measured from the top of the hook to the bottom of the drape. (It is sometimes referred to as the 'drop'.)
Because the drapery pins or hooks are usually set down from the top of the drapes, the Finished Length is longer than the Hook finish. How much longer depends on what heading depth you want above the hook.
If you want the drapes to cover the track when they are pulled together, then you might want a heading depth of 2". So your Finished Length will be the Hook Finish plus 2".
The Finished Length plus the hem/heading allowance is known as the Cut length, because this is the amount you cut off to make your drapes.
Allow 9" (23cms) extra for your hems and headings. So Cut Length will be the Finished Length plus 9" (23cms). (But see the section below on cutting patterned fabrics.)
Remember, always decide on the exact style of drape before you work out the fabric quantity. Heading allowances make a difference, especially to patterned fabrics.
You can now work out the drapery yardage!
Add the Finished length and Hem/Heading allowance together, then multiply the result by the Number of widths.
Just one extra stage to the calculations. When you add together the Finished length plus Hem/heading allowance, divide the result by the Pattern repeat, and round up to the nearest whole number. The Pattern repeat multiplied by this value is the cut length.
Work out a few examples and you'll soon get the hang of it!
Returns for cornice box or valance
Take the width of the supporting board, and add twice the return value (distance from front of board to the wall). That's the finished heading value. Keep the fullness the same as for the curtains. Then calculate the number of widths as before.
For the cut length, take the longest part of the valance and add 6" (7.5cms) for hem and heading allowances.
Take the finished width (front of board plus two returns) and add 6" (turning allowance). Divide this by the useable width of the fabric, and round up the result to get the number of widths.
The cut length will be the deepest measurement of the cornice plus 6" (7.5cms). Multiply the cut length by the number of widths to get the amount of fabric.
Calculating drapery yardage can seem intimidating if you haven't done it before. Just take it one step at a time.
It also helps to read through the section on how to make drapes. Once you understand the process, it makes working out the fabric for drapes much more logical.